Semiconductor light emitters are well-known and fall broadly into two categories: light emitting diodes (LEDs) and semiconductor lasers. The applications of these devices are numerous. LEDs are used extensively as displays, warning lights and indicating lights. Semiconductor lasers find wide application in the communications fields, such as for transmitting signals down optical fibers, writing information on compact discs and for use in projection televisions. While LEDs and semiconductor lasers are both light emitters, in many applications the two different devices are not interchangeable. Light emitting diodes are the devices of choice for many display applications. The low operating current, low power consumption, dispersal of light, and typical low cost of manufacturing are all advantages that light emitting diodes have over lasers for displays. They exhibit very long lives and maintain high efficiency and thus, have begun to replace many incandescent lamps in a number of applications . Semiconductor lasers, on the other hand, find wide application where coherent beams of light are required as described above.
While versatile and wide in their application, LEDs and semiconductor lasers suffer one serious deficiency; the wide variety of selection among them is severely limited in the green or blue wavelengths. While yellow and red semiconductor light emitters are popular and well-developed, blue and green light emitters have remained elusive. Blue and green light emitters would lend themselves to numerous applications. They would provide for advanced displays in the blue and green, where the human eye is most sensitive. They would provide the third primary color, the other two being red and yellow, whereby any color can be generated by combining those primaries in particular combinations. Also, because seawater shows the lowest absorption in the blue and green wavelengths, a blue or green semiconductor laser would provide an underwater optical communication means not currently available. In the recording industry, the density of information that may be recorded on an optical disc is currently limited by the wavelengths of the laser. These densities could be greatly increased, by a full order of magnitude, with the development of an inexpensive compact blue light emitter.
The mechanisms by which semiconductors emit light have been well-studied and are fairly well understood. LEDs and semiconductor lasers emit light as a result of electronic excitation of a material. An electron in an excited energy state, upon relaxing to a lower energy state, can emit a photon corresponding to the energy difference between the excited state and the lower energy state. The methods of exciting electrons vary, but for semiconductor light emitters the primary method is by injection electroluminescence. Energy is added to the system to coax electrons to a higher energy state. The energy states of concern in semiconductor light emitters can be characterized as the conduction band and the valence band.
Semiconductors are of three types, p-type, n-type and intrinsic. Intrinsic semiconductors can be made either p-type or n-type by introducing impurities, also called dopants of p-type or n-type, respectively. Semiconductor light emitting devices are essentially characterized by p-type material and n-type material having a pn-junction therebetween. Light emitting semiconductor devices have a recombination region between or within the p-type and n-type regions. At equilibrium, no light will be emitted by the device. However, if electrons from the n-type material can be coaxed into the conduction band over the holes of the p-type material, a situation arises where a number of electrons are excited. This coaxing is carried out by applying a forward bias across the junction. Electrons, once excited, after a period of time will relax from their excited energy level either spontaneously or by stimulation. This relaxation from the conduction band to the valence band often results in the emission of a photon.
The wavelength of an emitted photon will depend primarily on the energy difference between the conduction band and the valence band. This energy difference is referred to as the band gap of the material. The energy difference of the band gap is inversely related to the wavelength of emitted light by the well-known formula E.sub.g =hc/.lambda. where h is Planck's constant and c is the speed of light. Blue and green light is light of shorter wavelengths than red or yellow light. Therefore, to emit blue or green light requires a greater energy difference between the conduction band and valence band of the materials used. Red and yellow light emission results from a band gap in the range of 1.77 to 2.16 eV. Green and blue emission requires a band gap in the range of about 2.2 to 2.9 eV and beyond that to go into the violet. Sometimes the emitted light is slightly less energetic than the value E.sub.g, because carriers transition between shallow energy levels near the band edges.
Thus, to use direct emission of light as an approach to blue/green semiconductor light emitting devices, one has to obtain a band gap that exhibits the appropriate energy difference. Further, one has to be able to construct electronic devices with both n-type and p-type doping and to make appropriate electrical contact for easy conversion of current to emitted photons. A quick survey of the periodic table in the semiconductor region indicates that three primary groups of semiconductors are potentially useful for this application: the so-called III-V semiconductors in which the compound is made of an element from column III and an element from column V, the II-VI compounds in which the compound is made up of an element from column II and an element from column VI, and the compounds from the elements of column IVa of the periodic table.
For the III-V semiconductors, those with relevant band gaps for the blue and green involve either GaP in the green or GaN in the ultraviolet. GaP, while it can be doped both p-type and n-type, suffers the problem that it has an indirect band gap. An indirect band gap makes it difficult to produce high efficiency light emitting devices because the assistance of a phonon is necessary carry out the required electron transitions. To avoid this difficulty, manufacturers utilizing such materials have had to resort to quaternary alloys of AlGaInP to obtain direct transitions and this fix is only good to 2.30 eV in the green. Furthermore, compositions in quaternary systems are very difficult to control. On the other hand, compounds of GaN are extremely difficult to dope p-type and hence, there has been great difficulty in making pn-junction light emitting devices from them.
Examination of the properties of the II--VI compounds indicates that the materials with the appropriate band gaps for emission in the green and blue are ZnTe, ZnSe, ZnS and CdS, though CdS incurs the problems of ZnS without offering a large band gap like ZnS. All of these compounds are direct band gap materials and hence, suitable for making light emitters. Again, it has been difficult to obtain doping of the required levels in these systems for making light emitting pn-junctions. Notably, ZnTe can be doped p-type but it is extremely difficult to dope n-type. ZnSe has been historically easily doped n-type but difficult to dope p-type, and the sulfides have been relatively easily doped n-type but difficult to dope p-type.
Semiconductors produced by elements from group IV are limited. The only material of practical interest is SiC. Some crystal forms of this substance have band gaps that put them in the blue and can be doped to p- and n-type. However, difficulties controlling crystal quality have resulted in only low efficiency light emitters.
Recent attempts at producing blue and green light emitters have focused heavily on two areas. One is the bulk doping of p-ZnSe. Although blue light emission lasers have been reported, the problems inherent in bulk doping are significant. Another approach has been the utilization of heterojunctions, particularly ZnTe/ZnSe. This heterojunction configuration suffers from a large lattice mismatch of 7%, and neither material can be doped both types.
Accordingly, there is room for much advancement in the quest for blue and green semiconductor light emitting devices that solve the problems of the existing state of technology.